This invention relates to a valve. It relates more particularly to an improved gate valve of the type wherein a piston or slab-type gate intercepts and blocks inlet and outlet openings in the valve housing to close the valve.
Gate valves of the type with which we are concerned here are used particularly in high fluid pressure applications. For example, they are used to control the flow of oil in oilfields and of water in hydroelectric installations. In one type of gate valve, the gate itself is solid and when the valve is closed, the gate blocks aligned inlet and outlet openings in the valve seats. In another type, the gate has a transverse passage through it situated so that, when the valve is in its open position, the gate passage is in register with the inlet and outlet openings in the valve seats and when the valve is closed the gate passage is out of register with those openings being separated therefrom by appropriate seals. The latter type of gate valve has an advantage over the former in that the gate is always positioned between the valve seats and the respective seals thereby protecting the seats and seals from fluid flow while the valve is in the open position.
However, both gate valve types are disadvantaged in that fluid flow is closed off by the movement of the gate across a circular inlet or outlet seat opening, which opening is usually surrounded by a seal. Consequently, fluid flow attains a very high velocity right at the point of closing, the point being formed by the near eclipse of the seat opening and the gate or gate opening. In other words, as the gate reaches its closed position, it intercepts the inlet or outlet seat opening, making it almost a point opening which creates a pressure drop of maximum magnitude at that point. The high velocity fluid flow through that very small opening tends to erode and damage the valve seat and gate surfaces so that subsequent opening and closing of the valve moves the damaged metal surfaces over the seals between the gate and the valve seats thereby damaging the seals and degrading the sealing ability of the valve.
Some valves are designed to minimize seal damage due to the aforesaid effects. For example, one type of valve of which I am aware sold under the name Cameron Type F provides for valve seat rotation with each valve closure so as to minimize the damage to a singular point or location on the seals. While that approach mitigates the problem and enlarges the time between required valve parts replacement and repair, it does not solve the problem. Moreover, that type valve is relatively expensive to manufacture.